1. Field of the Invention
This invention relates to a control system for a power conversion system, and more particularly to a control system for a power conversion system used in, such as an interconnection system, composed of a plurality of power converters for converting AC power into DC power or DC power into AC power, connected between a plurality of AC power systems and a DC line, for exchanging electric power between the AC power systems and the DC line.
2. Description of the Related Art
FIG. 18 is a schematic block diagram showing one example of a main circuit of a conventional voltage source type self-commutated power conversion system (hereinafter referred to as a power conversion system). A power conversion system 100 is composed of a power converter 10, a DC capacitor 20, a linked reactor 30 and a converter transformer 40, and is connected between a DC line containing a DC power source 50 and an AC power system containing an AC system power source 60.
The principle of operation of power conversion system 100 composed of power converter 10, DC capacitor 20, linked reactor 30 and converter transformer 40 shown in FIG. 18, connected between a DC line and an AC power system for exchanging electric power between them is publicly known. For instance, it is described in pages 216 through 220 of "Semiconductor Power Conversion Circuit", an edition of the Semiconductor Power Conversion System Research Specialized Committee of the Institute of Electrical Engineers of Japan (the first edition was published on Mar. 31, 1987).
FIG. 19 shows one example of a main circuit of power converter 10. This circuit is composed of a plurality of bridge-connected self-turn-off devices (6 pieces here), for instance, gate turn-off thyristors GU, GV, GW, GX, GY and GZ, and diodes DU, DV, DW, DX, DY and DZ connected in anti-parallel to these thyristors, and is provided with DC terminals PT and NT, and AC terminals R, S and T.
FIG. 20 shows one example of a conventional control system of a power conversion system 100, and the same portions as shown in FIGS. 18 and 19 are assigned with the same reference numerals and the explanation thereof will omitted. A current transformer 45 detects AC current i flowing between converter transformer 40 and AC system power source 60. A potential transformer 46 detects AC voltage v applied at between converter transformer 40 and AC system power source 60.
A power detector (PQ detection) 70 detects an active power Pd and a reactive power Qd by AC current i detected by current transformer 45 and AC voltage v detected by potential transformer 46.
A power control system 80 is composed of an active power reference setter 71, a reactive power reference setter 72, an active power controller (APR) 73, a reactive power controller (AQR) 74, a voltage phase detector (PLL) 75, a constant current control circuit (ACR) 76, and comparators 77, 78.
Comparator 77 compares an active power reference Pdp that is set by active power reference setter 71 with active power detected value Pd that is detected by power detector 70 and obtains a difference between them. Active power controller 73 inputs from comparator 77 the difference between active power reference Pdp and active power detected value Pd and outputs an active current command value Ipref so as to minimize the difference.
Comparator 78 compares a reactive power reference Qdp that is set by reactive power reference setter 72 with reactive power detected value Qd that is detected by power detector 70 and obtains a difference between them. Reactive power controller 74 inputs from comparator 78 the difference between reactive power reference Qdp and reactive power detected value Qd and outputs a reactive current command value Iqref so as to minimize the difference.
Constant current control circuit (ACR) 76 receives AC current i detected by current transformer 45, AC voltage v detected by potential transformer 46, a system phase .theta. detected by voltage phase detector 75, and acts to coincide system current detected value i to active current command value Ipref that is output from active power controller 73 and reactive current command value Iqref that is output from reactive power Controller 74 using system phase signal .theta. detected by voltage phase detector 75 and system voltage signal v, and outputs output voltage command values Vuc, Vvc and Vwc.
The principle of constant current control circuit 76 is disclosed in Japanese Patent Publication (Kakai) No. Hei 1-77110. The examples of constant current control circuit 76 are disclosed in a literature titled "Application of a digital instantaneous current control for static induction thyristor converter in the utility line", PCIM Proceeding (Dec. 8, 1988) by Shun-ichi Hirose, et al. and others and therefore, the detailed explanation is omitted here.
A gate control circuit 90 decides a firing pattern (ON/OFF timing) for each device of power converter 10 according to output voltage command values Vuc, Vvc and Vwc that are output from constant current control circuit 76.
FIG. 21 shows an example of a control system of a power conversion system, where DC power source 50 in FIG. 20 is constructed by a voltage source type self-commutated power conversion system 100B which is almost in the same construction as voltage source type self-commutated conversion system 100, and the same reference numerals suffixed with A or B are assigned to the same parts as in FIG. 20 and the explanations thereof are omitted.
In power conversion system 100B, a comparator 84 obtains a difference between DC voltage reference Edp set by a DC voltage reference setter 79 and DC voltage detected value Ed detected by DC voltage detector 21. A DC voltage controller (AVR) 81 inputs from comparator 84 the difference between DC voltage reference Edp and DC voltage detected value Ed and outputs active current command value Ipref so as to minimize the difference.
When power conversion systems 100A and 100B are constructed as explained above, it becomes possible to interchange electric power as desired between an AC system power source 60A and an AC system power source 60B by setting and controlling DC voltage reference Edp and DC voltage Ed in power conversion system 100B and by setting and controlling active power reference Pdp and active power Pd in power conversion system 100A.
In the construction as shown in FIG. 21, it becomes no longer possible to maintain DC voltage Ed, if power conversion system 100B which is controlling DC voltage Ed becomes faulty and stops to run, or if AC system power source 60B causes such troubles as ground fault and power conversion system 100B stops to run. If stopped power converter 100B is operated as a rectifier, it becomes not possible to supply power and DC undervoltage is generated. Further, if power conversion system 100B is operated as an inverter, it becomes not possible to consume power and DC overvoltage is generated.
As described above, according to the conventional art, there are such defects that when the DC sides of power conversion systems are connected for interchanging electric power between two AC power systems, if one power conversion system stops to run for the fault of the power conversion system or for ground fault of the power system, DC overvoltage or DC undervoltage are generated, and as a result, the other normal power conversion system also stops to run.
In particular, when electric power is interchanged by more than 3 power conversion systems with the DC sides of more than 3 power conversion systems connected, there is such a defect that when one power conversion system stops to run although it is possible to interchange electric power by two normal power conversion systems, it becomes not possible to interchange power as the normal power conversion systems also stop to run because of DC overvoltage or DC undervoltage.
In a system used practically shown in FIG. 21, considering also the case that power conversion system 100A controls the DC voltage and power conversion system 100B controls the active power, a DC voltage reference Setter 79A, a comparator 84A and a DC voltage controller 81A are provided in power conversion system 100A, and an active power reference setter 71B, a comparator 77B and an active power controller 73B are provided in power conversion system 100B, though not shown in FIG. 21.